Continuous process and apparatus for the drying and gel formation of solvent-containing gel-forming polymers

ABSTRACT

A continuous process for the drying and gel formation of solvent-containing gel-forming polymers, in particular polysaccharide derivatives, by flash cooling of suspensions of the polymers, and an apparatus for the drying of solvent-containing products are described. The process comprises the steps: metering the solvent-containing polymer having a solids content of 1 to 65% by weight and at a pressure of in particular ambient pressure to 6000 hPa, at a polymer temperature of 20 to 100° C. into an evaporation zone ( 1 ), the evaporation zone ( 1 ) having a pressure of 0.1 hPa to 800 hPa, cooling the polymer, in particular to a temperature of &lt;93° C., freeing the polymer of 2 to 5% by weight of solvent with mechanical stirring, mixing and conveying of the mixture to the lower end of the evaporation zone ( 1 ) with formation of a dense gelled product layer, discharging the polymer by means of a mechanical discharge apparatus ( 7 ), optionally with further densification of the polymer, and subsequently drying the gelled polymer in a drying apparatus to a solvent content of not more than 5% by weight.  
     The apparatus comprises at least one evaporation container ( 3 ) having an evaporation zone ( 1 ), a stirrer and a conveyor means ( 8 ), a product inlet ( 12 ), a vapor take-off ( 5 ) with connected vacuum generator, and a discharge apparatus ( 7 ) connected to the lower end of the evaporation container ( 3 ), one or more conveyor paddles ( 9 ) having thrust blades ( 10 ) which pass across the entrance of the discharge apparatus ( 7 ) at a height of not more than 5 mm, in particular 1 to 3 mm, being provided in the lowermost plane of the conveyor means ( 8 ).

[0001] The invention relates to a continuous process for the drying andgel formation of solvent-containing gel-forming polymers, in particularpolysaccharide derivatives, by flash cooling of suspensions of thepolymers and to an apparatus for the drying of solvent-containingproducts.

[0002] The process comprises the steps: metering the solvent-containingpolymer having a solids content of 1 to 65% by weight and a pressure ofin particular ambient pressure to 6000 hPa, at a polymer temperature of20 to 100° C., into an evaporation zone, the evaporation zone having apressure of 0.1 hPa to 800 hPa, cooling the polymer, freeing the polymerof 2 to 5% by weight of solvent with mechanical stirring, mixing andconveying of the mixture to the lower end of the evaporation zone withformation of a dense gelled product layer, discharging the polymer bymeans of a mechanical discharge apparatus, optionally with furtherdensification of the polymer, and subsequently drying the gelled polymerin a drying apparatus to a solvent content of not more than 5% byweight.

[0003] The apparatus described comprises at least an evaporationcontainer having an evaporation zone, a stirrer and a conveyor means, aproduct inlet, a vapour take-off with connected vacuum generator, adischarge apparatus connected to the lower end of the evaporationcontainer, one or more conveyor paddles having thrust blades which passacross the entrance of the discharge apparatus at a height of not morethan 5 mm, preferably 1 to 3 mm, being provided in the lowermost planeof the conveyor means.

[0004] German Offenlegungsschrift 196 02 227 A1 discloses a storagecontainer (paste bunker) having a discharge apparatus. In particular,the described stirrer of the storage container has a sleeve design withvariable stirring arm geometry for adaptation to the productconsistency.

[0005] German Patent Specification 952 264 discloses a three-stageprocess for converting moist, fibrous methylcellulose into powder formof high dissolution rate. Here, the product containing 50 to 70% byweight of water is first homogenized to give a plastic material and iscooled to 10 to 20° C., a cooled screw press being used; the material isthen milled using a hammer mill and is dried in a forced-circulationdryer. It is the object of the invention to provide a process and anapparatus which avoid the disadvantages of the known processes andapparatuses and permit comparably safer continuous operation.

[0006] Prior gelling of the cellulose ethers is necessary for the dryingand for the millability of the cellulose ethers in a miller-dryer.

[0007] In particular, it is therefore the object of the invention todevelop a drying process which is suitable for solvent-containinggel-forming polysaccharide derivatives, in particular for a celluloseether suspension, it being necessary to cool the cellulose ethersuspension in order to effect gelling and to meter the gel continuouslyinto a downstream apparatus, in particular into a miller-dryer.

[0008] In particular, a cellulose ether suspension must be capable ofbeing converted into a homogeneous, gel-like consistency in order to beable to meter the material into a superheated steam miller-dryer.

[0009] The object is achieved, according to the invention, if thepreheated polymer suspension is introduced into an evaporation zone andthe evaporation zone, for example a cylindrical container, is presentunder a reduced system pressure and the polymer suspension experiences acooling effect down to below the flocculation point as a result offlashing into the cylindrical container.

[0010] The invention relates to a continuous process for the drying andgel formation of solvent-containing gel-forming polymers, in particularpolysaccharide derivatives, particularly preferably of hot, aqueouscellulose ether suspensions, by flash cooling (abrupt let-down withcooling) of suspensions of the polymers, comprising the steps: meteringthe solvent-containing polymer having a solids content of 1 to 65% byweight, preferably of 5 to 50% by weight, particularly preferably of 10to 40% by weight, at a pressure of in particular ambient pressure to6000 hPa, at a polymer temperature of 20 to 100° C., preferably of 30 to95° C., into an evaporation zone, preferably a paste bunker, theevaporation zone having a pressure of 0.1 hPa to 800 hPa, preferably of20 hPa to 700 hPa, particularly preferably of 40 hPa to 200 hPa, coolingthe polymer, in particular to a temperature of <93° C., in particular<60° C., especially of 60° C. to 29° C., freeing the polymer of 2 to 5%by weight of solvent with mechanical stirring, mixing and conveying ofthe mixture to the lower end of the evaporation zone with formation of adense gelled product layer, discharging the polymer by means of amechanical discharge apparatus, in particular with the aid of asingle-screw or multiscrew conveyor, optionally with furtherdensification of the polymer, and subsequently drying the gelled polymerin a drying apparatus to a solvent content of not more than 5% byweight.

[0011] The residence time of the polymer in the evaporation zone ispreferably from 30 sec to 2 h, particularly preferably from 5 to 30 min.

[0012] Preferably, the drying of the gelled polymer is effected at 100to 500° C., particularly preferably from 100 to 230° C., in amiller-dryer.

[0013] Advantageously, the drying of the gelled polymer is carried outin particular at a pressure of about 1000 hPa (i.e. ambient pressure) to5000 hPa, preferably at 1000 hPa.

[0014] Particular advantages are obtained if the drying of the gelledpolymer is effected in a superheated steam miller-dryer.

[0015] Preferably, the drying of the gelled polymer is effected to asolvent content of not more than 4% by weight.

[0016] The polymer particularly preferably to be used in the process isa cellulose ether, in particular methylcellulose.

[0017] The solvent used in the process is preferably essentially water.

[0018] The invention is distinguished from the prior art, in particularthat described in German Patent Specification 952 264, in that thecooling of the polymer suspension takes place not by contact cooling ofthe suspension but by the flash effect, i.e. during the transition fromthe higher pressure level in the suspension line to the reduced systempressure in the paste bunker (evaporation zone). In contrast to thescrew press, this process takes place spontaneously with simultaneousconcentration of the polymer suspension. As described in the presentpatent; the further gelling is achieved by the available residence timein the paste bunker. In contrast to the screw press, the paste bunkeradditionally has the desirable function of a buffer. The gelling of thepolymer (methylcellulose) takes place at a temperature of 10 to 20° C.according to the teaching of German Patent Specification 952 264 but inthe present process it takes place at a substantially highertemperature, but typically at <60° C.

[0019] The invention furthermore relates to an apparatus for the dryingof solvent-containing products, in particular of solvent-containinggel-forming polymers, comprising at least one evaporation containerhaving an evaporation zone, a stirrer and a conveyor means, a productinlet, a vapour take-off with connected vacuum generator, and adischarge apparatus connected to the lower end of the evaporationcontainer, one or more conveyor paddles having thrust blades which passacross the entrance of the discharge apparatus at a height of not morethan 5 mm, preferably 1 to 3 mm, being provided in the lowermost planeof the conveyor means.

[0020] The discharge apparatus is preferably a single-screw ormultiscrew conveyor.

[0021] The evaporation container has an additional jacket for passingthrough a heat-transfer medium in order additionally to promote thedevolatilization of the polymer.

[0022] In a particularly preferred variant of the apparatus, the thrustblades pass across the entrance of the discharge apparatus at a heightof not more than 5 mm, preferably 1 to 3 mm, in order to ensure completefilling of the screw flights. Uniform product conveying and at the sametime a tight connection between paste bunker and miller-dryer are thusachieved.

[0023] In a preferred embodiment of the apparatus, a throttle means, inparticular a throttle valve for maintaining the reduced pressure in theevaporation container, is mounted upstream of the product inlet.

[0024] A solid-blade screw conveyor has proved particularly useful asthe discharge apparatus.

[0025] The apparatus according to the invention is particularly suitablefor carrying out the above-described process according to the invention.

[0026] Compared with the arrangement disclosed in German Specification195 02 227 A1, the paste bunker in the apparatus according to theinvention has in particular a thrust part, additionally screwed on ineach case, on the lower arms, for metering the polymer gel.

[0027] The preferred paste bunker shown in FIG. 1 permits gas-tightcontinuous metering into a downstream drying apparatus. The flashcooling makes use of this advantage in order continuously to flash (tolet down) solvent-containing polymers, in particular polysaccharidederivatives, particularly preferably cellulose ether suspensions, havinga solids content of 1 to 65% by weight, preferably of 5 to 50% byweight, particularly preferably of 10 to 40% by weight, via anadjustable valve into the cylindrical container under a pressure of 0.1hPa to 800 hPa, preferably of 20 hPa to 700 hPa, particularly preferablyof 40 hPa to 500 hPa.

[0028] The flash effect takes place spontaneously when the suspensionenters the evaporation zone, if the suspension temperature is above thesaturated vapour temperature of the solvent, which temperature formspart of the system pressure. Particularly for cellulose ethers, thesystem pressure is chosen so that the saturated vapour temperature is<60° C. (below the flocculation point). The vapours formed during theflash effect (flash evaporation) leave the paste bunker via a connectionin the lid of the cylindrical container. Preferably, the suspension isconcentrated by 2 to 5% and the polymers are cooled to <60° C., inparticular 60° C. to 29° C. In order to avoid condensation of thevapours on the cylindrical outer wall, these can preferably be designedto be heatable by means of a double jacket. In order to achieve anadditional cooling effect after the flash process, the double jacket canin particular also be cooled, the concentration effect being reduced bythe flashing of the solution due to partial condensation.

[0029] The paste bunker gives a sufficient residence time, adjustable bymeans of the height of fill, in order to ensure complete gelling beforethe gel leaves the paste bunker through the conveyor screw. The poorlymeterable product, in particular cellulose ether gel, is fed to theconveyor screw with the aid of a stirrer and the thrust parts mounted inparticular in the lowermost stirring plane.

[0030] Owing to its design, the paste bunker permits gas tightness withrespect to downstream apparatuses, in particular with respect to adrying step by means of superheated steam, so that the state of reducedsystem pressure in the bunker results in continuous flashing of the hotcellulose ether suspension with simultaneous cooling to the saturatedvapour temperature forming part of the system pressure. The systempressure is set so low that the saturated vapour temperature is belowthe flocculation point of the cellulose ether suspension. At the sametime, concentration of the suspension is achieved by the flash effect.On cooling, a homogeneous gel is produced. The residence time forcomplete gelling can be adjusted in the bunker. The paste bunkersimultaneously performs the function of metering the gel into thesuperheated steam miller-dryer.

[0031] The use of a paste bunker having a modified sleeve stirrer isparticularly suitable for achieving the technical objects (gentle gelformation) of the invention. The cooling of the hot polymer suspensionis achieved by “flashing” into the paste bunker subjected to a vacuum,e.g. at <200 mbar. At the same time, concentration of the celluloseether suspension and gelling occur. Vapours formed are taken off via thevapour connection in the lid of the paste bunker. Gelling is permittedby a sufficient product residence time, adjustable by means of the levelin the paste bunker. The preferably used solid-blade screw meters thegel into the downstream miller-dryer, in particular into a superheatedsteam miller-dryer.

[0032] The design of the stirring arms, in particular of the lowerplane, can preferably be adapted to the conveying behaviour of theproduct and is all the more important for ensuring gas-tight,continuous, uniform metering.

[0033] The point for feeding the suspension into the paste bunker ispreferably such that feeding is effected mainly tangentially into thepaste bunker for good separation of the suspension from the vapours.However, the flashing may also be effected in particular via the lid ofthe paste bunker. The height of the feed point can be variable. In orderto avoid condensation of the vapours, the paste bunker can, asdescribed, be heated by means of a double jacket. In order to assist thecooling effect during flashing of the suspension, the paste bunker maybe cooled by means of the double jacket. In this case, the achievableconcentration is reduced by condensing vapours.

[0034] Inter alia, the following advantages are achieved by theinvention:

[0035] 1. Gentle cooling/drying of suspensions, in particular ofsolvent-containing gel-forming polysaccharide derivatives, especially acellulose ether suspension, by the flash effect.

[0036] 2. Cooling of solvent-containing, gel-forming polysaccharidederivatives, in particular of the cellulose ether suspension, below theflocculation point.

[0037] 3. Separation of the resulting vapours from the continuouslyflashed-in suspension.

[0038] 4. Sufficient residence time for gelling, adjustable by thevariable level in the paste bunker.

[0039] 5. Formation of a homogeneous gel on cooling and gelling.

[0040] 6. Continuous gas-tight metering from the paste bunker into thedownstream drying apparatus, in particular into a superheated steammiller-dryer.

[0041] 7. Stirrer geometry adaptable to the product consistency in orderto ensure constant gas-tight metering.

[0042] The invention is further explained below with reference to thefigures and by means of the examples, which, however, do not representany restriction of the invention.

[0043]FIG. 1 shows a schematic diagram of the flash introduction of thepolymer

[0044]FIG. 2 shows a basic design of the preferably used paste bunker

EXAMPLES General Description

[0045] A methylcellulose suspension prepared in a moist mixer (notshown) is metered continuously with the aid of a Mohno pump (not shown)into the paste bunker 1 under a reduced system pressure. The level inthe paste bunker 1 is kept constant by adaptation of the Mohno pumpspeed to the gel throughput established by the conveyor screw 7. Theconveyor screw 7 of the paste bunker 1 conveys the gel into thesuperheated steam miller-dryer 2 (Ultra-Rotor II type, manufacturerJaeckering), where it is dried and milled. The vapours formed onflashing of the methylcellulose suspension are extracted with the aid ofa vacuum pump 13 via a valve 19 and the connection 5 in the lid 6 of thepaste bunker 1. The system pressure in the paste bunker 1 is set andkept constant by means of a pressure relief valve 18. A vent valve 20 isalso mounted on the same feed line. In order to avoid crossing of thevacuum into the suspension line 12 and gelling of the suspension withthe danger of blockage of the suspension line 12, a ball valve 4 isinstalled before the entrance into the cylindrical part of the pastebunker 1. The inlet is arranged tangentially to the housingcircumference. The pressure in the suspension line 12 is set byadjusting the ball valve 4 at >100 mbar superatmospheric pressure. Aninlet 15 and an outlet 16 for a heating fluid are provided on the pastebunker 1.

Embodiment

[0046] The stirrer 8 of the paste bunker 1 is equipped with fourstirring planes angled so that they press downwards and three retarders14 which are fastened in the cylindrical wall 3 and prevent co-rotationof the product with the stirrer 8. The two lowermost stirring arms 9 ofthe lowermost stirring plane are additionally equipped, in the outerregion over a length corresponding to the screw trough width, with awedge-shaped thrust part 10. These thrust parts 10 are 2 mm away fromthe bottom and ensure continuous metering of the product to the conveyorscrew 7 by the stirrer 8.

[0047] A 21% strength by weight methylcellulose suspension (product MT40000, producer Wolff-Walsrode GmbH) at 82° C. was flashed (metered)continuously into the paste bunker 1 with the aid of a Mohno pump. Thesystem pressure in the paste bunker 1 was 130 mbar. The pressure in thesuspension line 12 was set at 3 bar superatmospheric pressure with theaid of the ball valve 4. The double jacket 11 of the paste bunker 1 wasnot heated and not cooled. The concentration of the methylcellulosesuspension increased in the moist mixer to 23.5% by weight by removal ofwater by evaporation.

[0048] Throughputs of moist material in the range from 57 to 165 kg/h ofsuspension were operated. The residence time in the paste bunker 1 was14 min at low throughput and 6 min at high throughput. The concentrationby the flashing is about 0.054 kg of solvent per kg of suspension. Onincreasing the throughput, the stirrer speed was increased from 15 to 27rpm and the conveyor screw speed from 12 to 28 rpm. The conveyor screw 7conveyed the gel into the miller-dryer 2. By increasing the superheatedsteam inlet temperature in the miller-dryer 2 from 133 to 181° C. athigh throughput, the superheated steam outlet temperature of 120° C. waskept constant. The dried product had a residual moisture content of 2%.The system pressure was 10 mbar superatmospheric pressure on entry ofthe superheated steam into the miller-dryer 2 and 10 mbar reducedpressure on exit from the dryer 2. The speed of the miller-dryer 2 wasset at 1063 rpm.

1. Continuous process for the drying and gel formation ofsolvent-containing gel-forming polymers, in particular polysaccharidederivatives, particularly preferably of hot, aqueous cellulose ethersuspensions, by flash cooling of suspensions of the polymers, comprisingthe steps: metering the solvent-containing polymer having a solidscontent of 1 to 65% by weight, preferably of 5 to 50% by weight,particularly preferably of 10 to 40% by weight, at a pressure of inparticular ambient pressure to 6000 hPa, at a polymer temperature of 20to 100° C., preferably of 30 to 95° C., into an evaporation zone (1),preferably a paste bunker, the evaporation zone (1) having a pressure of0.1 hPa to 800 hPa, preferably of 20 hPa to 700 hPa, particularlypreferably of 40 hPa to 0.200 hPa, cooling the polymer, in particular toa temperature of <93° C., in particular <60° C., especially of 60° C. to29° C., freeing the polymer of 2 to 5% by weight of solvent withmechanical stirring, mixing and conveying of the mixture to the lowerend of the evaporation zone (1) with formation of a dense gelled productlayer, discharging the polymer by means of a mechanical dischargeapparatus (7), in particular with the aid of a single-screw ormultiscrew conveyor (7), optionally with further densification of thepolymer, and subsequently drying the gelled polymer in a dryingapparatus to a solvent content of not more than 5% by weight.
 2. Processaccording to claim 1, characterized in that the residence time of thepolymer in the evaporation zone (1) is from 30 sec to 2 h, preferablyfrom 5 to 30 min.
 3. Process according to claim 1 or 2, characterized inthat the drying of the gelled polymer is effected at 100 to 500° C.,preferably from 100 to 230° C., in a miller-dryer.
 4. Process accordingto any of claims 1 to 3, characterized in that the drying of the gelledpolymer is effected at a pressure of 1000 to 5000 hPa, preferably of1000 hPa.
 5. Process according to any of claims 1 to 4, characterized inthat the drying of the gelled polymer is effected in a superheated steammiller-dryer.
 6. Process according to any of claims 1 to 5,characterized in that the drying of the gelled polymer is effected to asolvent content of not more than 4% by weight.
 7. Process according toany of claims 1 to 5, characterized in that the solvent is essentiallywater.
 8. Process according to any of claims 1 to 5, characterized inthat the polymer is a cellulose ether, in particular methylcellulose. 9.Apparatus for the drying of solvent-containing products, in particularof solvent-containing gel-forming polymers, comprising at least oneevaporation container (3) having an evaporation zone (1), a stirrer anda conveyor means (8), a product inlet (12), a vapour take-off (5) withconnected vacuum generator, and a discharge apparatus (7) connected tothe lower end of the evaporation container (3), one or more conveyorpaddles (9) having thrust blades (10) which pass across the entrance ofthe discharge apparatus (7) at a height of not more than 5 mm, inparticular 1 to 3 mm, being provided in the lowermost plane of theconveyor means (8).
 10. Apparatus according to claim 9, characterized inthat the discharge apparatus (7) is a single-screw or multiscrewconveyor (7).
 11. Apparatus according to claim 9 or 10, characterized inthat the evaporation container (3) has an additional jacket (11) forpassing through a heat-transfer medium.
 12. Apparatus according to anyof claims 9 to 11, characterized in that the thrust blades (10) passacross the entrance of the discharge apparatus (7) at a height of notmore than 2 mm.
 13. Apparatus according to any of claims 9 to 12,characterized in that a throttle means (4), in particular a throttlevalve (4) for maintaining the reduced pressure in the evaporationcontainer (3), is mounted upstream of the product inlet (12). 14.Apparatus according to claim 10, characterized in that a solid-bladescrew conveyor (7) is provided as the discharge apparatus (7). 15.Apparatus according to any of claims 9 to 14 for carrying out theprocess according to any of claims 1 to 8.